I am probably going to repeat earlier remarks but what the hell.
1. Give the part of the header which contains the board title its own class e.g. .punlogo. The reason is to make it easier for site integration. Users could insert a background graphic or their site logo via the stylesheet without having to edit main.tpl. Even without a graphic somebody might want to make the board header a different colour from the rest of the board. The default setting would be the same as at present ie same as .punhead.
2. .punspacer is overused. Reserve it for empty spacing and use a new class .punfilledspace for whitespace which contains text/links. cellspacing and cellpadding on empty white space elements should be set to 0. That way the height of empty spacing could be set exactly (even to nothing) as there would be nothing to restrict the height adjustment.
3. Not strictly CSS, but related. Put the admin interface on it's own template e.g. admin.tpl which will in fact be a copy of the standard main.tpl and have it use one of the default styles. The reason is it is easier to integrate the board into a site if you don't have to worry about admin. It also speeds up the admin interface since it is using the standard template rather than having to sit in the middle of some three column portalized thingy. It takes one variable at the top of common.php and common_admin.php and a couple of if{} statements to do this. I don't think this change could be considered a move towards templating, it's just separating the two distinct interfaces of the board. If people want to re-integrate the admin interface is just a matter of setting the value of the variable at the top of common_admin.php. Mind you, this is so simple to do I could always write some instructions as a mini mod.
4. There is some duplication of classes. At the moment you have puncon1 and puncon1right where the only difference is alignment. You also have punright. You should be able to double up classes e.g. <td class = "puncon1 punright">. Same goes to top and center alignments. that should enable you to get rid of three or four classes altogether. As far as I know this works in all modern browsers but better ask Louis about that as there may be some problems I don't know about.
5. Put main navigation in a <div> and give it an id of #punnavbar. Wanting to style main navigation differently from the rest of the board is a fairly common thing and this would make it easier.